1893.] MICROSCOPICAL JOURNAL. 21 



and safely examined, and there is no immersion fluid to be cleaned 

 away from lens or cover-glass. The moderate angle allows a cer- 

 tain amount of penetration (thickness of focal plane) , which 

 enables one to perceive quickly the inter-relation of cells or other 

 structures, and such an objective is comparatively easy to handle 

 in every way. The unavoidable loss of light between the cover- 

 glass and front line of a dry objective materially reduces defini- 

 tion, i. e., resolution, and this considerably oflsets the convenience 

 attending its use and bars it out for critical work over the more 

 minute objects. 



An immersion objective, utilizing more light than a dry one, 

 presents a sharper and more brilliant image. A water immersion 

 objective of but 89° angle will transmit as much light as a dry 

 one of 140°. A good hom. imm. objective of 1.40 N. A. will 

 transmit nearly two and a half times more light than either. The 

 study of minute objects, like diatoms, the flagelhe and spores of 

 bacilli, very delicate membranes etc., requires the use of the 

 highest grade of immersion objectives. Diatoms, generally, are 

 symmetrical, colorless, and flat ; their markings are regular and have 

 comparatively sharp outlines, and their resolution is assisted by 

 the contrast of the shadows produced by oblique light. Slight 

 degrees of aberration in the correction of the objectives may be 

 detected by the color or distortion of the image. Perfect adjust- 

 ment by means of the collar can be rather easily made, and the 

 full powers of a good objective brought into use over these objects. 

 In histological work the conditions are difterent, and it is a more 

 troublesome matter to acquire the ability to correct a fine objec- 

 tive. Histological elements are of various sizes and shapes ; 

 they lie in all planes and directions. They are usually 

 highly colored (stained), and are, as a rule, examined with 

 the light central. The art of correcting a high-grade objec- 

 tive so as to secure its best performance for this work means the 

 mastery of a Moller test-plate with oblique light, and then a post- 

 graduate course on the same plate, until its No, 18 can be 

 resolved with central light. After this practice, work over histo- 

 logical elements can be intelligently undertaken. In skilled 

 hands a hom. imm. objective presents images so satisfying that 

 one would never willingly go back to the use of inferior lenses 

 for any kind of work. Histological sections or teasings are of 

 appreciable thickness, and a glass with a fair working distance is 

 a necessity. This in many cases precludes the use of objectives 

 of the highest angle, with their necessarily very short working 

 distance. The writer has a i-io inch of 1.32 N. A. which can 

 be used over the average mounted section, and it is very satisfac- 

 tory for work over a wide range of objects. Such a lens has but 

 a small amount of penetration, and the higher angle, 1.40 N. A., 

 has still less ; but frequent and slight changes of focus compen- 

 sates for this to the practised eye. S. G. Shanks, M. D. 



Albany, N. X^Jan. 22^ jSgj. 



